1. Field of the Invention
This invention relates generally to the circuit and methods for configuring and operating a switching power converter. More particularly, this invention relates to a bi-directional magnetization control circuit for magnetizing a magnetic amplifier for stabilizing an auxiliary output voltage of a power converter such that the converter can be applied to broader ranges of load imposed on the output terminals.
2. Description of the Prior Art
The magnetic amplifier is commonly implemented for controlling the auxiliary output voltage of a switching power converter. Referring to FIG. 1A for a forward power converter implemented with a magnetic amplifier in an auxiliary output power control. In a forward power converter, the energy is transferred from the primary side receiving an input voltage from a power source to the secondary side that carries a load. The energy transfer is achieved through the transformer under a condition of constant voltage. According to the duty cycle of the pulse width modulation (PWM) control circuit, a pulse width modulation (PWM) control performs the function of stabilizing the primary output voltage. Referring to FIG. 1B for more details of the operational cycles of the forward power converter. The current of the primary side of the transformer is ip. When the main switch S is turned off at a time point t3, the current ip drops down to zero, and the voltage Vg on the auxiliary output loop turns to a negative voltage between t3 to t1. During this time period, the magnetic amplifier control circuit carries out a magnetic reset of the magnetic amplifier. The total amount of magnetic reset energy stored into the magnetic amplifier is shown as the shadowed area having a volt-time integrated value of (V2xe2x88x92Vg)(t3xe2x88x92t1). At the time point t1, the main switch S is turned on, the voltage Vg turns into a positive voltage. Meanwhile, the magnetic amplifier, after the magnetic reset, has a high impedance. With the discharge current from the inductor L2, the diode D4 becomes conductive and the voltage V2 is clamped to a voltage of zero. The voltage Vg next to the magnetic amplifier performs a magnetizing function to the magnetic amplifier until the magnetic amplifier reaches a saturation state when Vg(t2xe2x88x92t1) is equal to (v2xe2x88x92Vg)*(t3xe2x88x92t1). When the magnetic amplifier reaches a saturation state, the magnetic amplifier presents a low impedance condition. The voltage V2 is raised to a level of Vg. For this reason, adjustment of a value of the magnetic reset volt-time integration, i.e., the value of (v2xe2x88x92Vg)*(t3xe2x88x92t1), of the magnetic amplifier can control the pulse width (t2xe2x88x92t1) of V2 and that can in turn control the output voltage. Under the condition when the load of the output loop implemented with the magnetic amplifier is increased, the output voltage is decreased with higher load. In order to compensate for the higher load to maintain a stable output voltage, the magnetic amplifier control circuit can increase the pulse-width of the voltage V2 by decreasing the time required for magnetic saturation. However, even if the magnetic amplifier has an output reset current of zero, a certain time duration would still be required for the magnetic amplifier to magnetize from a residual flux density Br to a saturation low-impedance condition as described above. A minimum magnetization time is therefore required and that minimum time duration can be represented as:
Tdelay=[N*Ae*(Bsxe2x88x92Br)]/Vexcitingxe2x80x83xe2x80x83(1)
Where Bs is the saturation magnetic flux density, Br is the residual magnetic flux density, N is the number of windings of the magnetic amplifier and Ae is a cross-section area of the magnetic amplifier. When the load of the power converter is small, the duty cycle of the main switch is also reduced. However, if the time duration in which the main switch is kept on is small than a minimum magnetization time duration Tdealy as that shown in Equation (1), the magnetic amplifier will never be magnetized to a saturation state. Under that condition, the auxiliary output voltage loop will not able to function properly and the auxiliary output voltage will be reduced thus adversely affect the stability of the output voltage of the power converter. In other words, for the purpose of assuring a stable output voltage, the duty cycle of the main switch must be maintained at a value higher than a minimum duty cycle to satisfy the requirement derived from Equation (1). For that reason, when the magnetic amplifier is implemented in an auxiliary output loop as that shown in FIGS. 1A and 1B, a minimum load must be maintained. The minimum load must be higher than a specific value in order to maintain the duty cycle of the main switch higher than a minimum duty cycle. Thus, application of the power converter becomes less flexible because it can only be used to handle a load that is limited to a predetermined range.
Therefore, a need still exists for those of ordinary skill in the art to provide a new and improved power converter implemented with magnetic amplifier for providing a stabilized auxiliary power that can resolve the above-discussed technical limitations.
It is therefore an object of the present invention to provide a novel and improved power converter implemented with control circuit for carrying out bi-directional magnetization for a magnetic amplifier to expand the range of load application of a forward converter employing a magnetic amplifier. A forward converter that enables a bi-directional magnetization of a magnetic amplifier significantly reduces the minimum magnetization time required for a magnetic amplifier. The converter thus allows a lower cycle duty of the main switch and that in turns expands the range of the load that is operable with the improved forward converter implemented with a magnetic amplifier. The new and improved forward converter implemented with a feature of bi-directional magnetization thus enable a person of ordinary skill in the art to resolve the above mentioned difficulties and limitations.
Specifically, magnetic amplifiers are implemented together with a pulse width modulation controller. The PWM controller is applied to control the main output voltage and the magnetic amplifiers are used to control the auxiliary output voltage such that stabilized output voltage is produced. For the purpose of expanding the operable range of the load processed by the magnetic amplifier, two magnetizing windings W1 and W2 are implemented with two control circuits are implemented to provide negative and positive magnetizing currents carry out a negative and positive magnetization. The limitation of keeping the load within a narrow range caused by inability to reach magnetic saturation condition as discussed above is therefore resolved.
Briefly, in a preferred embodiment, this invention discloses a power converter that includes a transformer for transferring an input voltage from a primary side to a secondary side. The secondary side includes a main output voltage loop and at least one auxiliary output loop connected with a magnetic amplifier. A pulse width modulation (PWM) controller controls a switch on the primary side of the transformer for turning on the switch and turning off the main output voltage loop and the auxiliary output voltage loop for storing a magnetizing energy on windings of the secondary side. The magnetic amplifier includes a first and a second magnetization windings controlled by a first and a second control circuits respectively for providing a positive and negative magnetization current to carrying out a bi-directional magnetization process to achieve expanded load range operable for the forward power converter.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.